1. Field of the Invention
The present invention relates to an abnormality diagnosis apparatus for a cooling system of a vehicle, which includes a coolant circuit that circulates coolant between an internal combustion engine and a radiator of the vehicle and is provided with a thermostat.
2. Description of Related Art
In general, in a cooling system for cooling an internal combustion engine installed in a vehicle, a thermostat (also referred to as a thermostat valve or a thermo valve) is provided in a coolant circuit that circulates coolant between the engine and a radiator. When the coolant temperature is lower than a predetermined temperature (e.g., a temperature of the coolant at a warm-up complete state of the engine), the thermostat is closed to stop the circulation of the coolant between the engine and the radiator. In this way, the coolant temperature at the engine side is rapidly increased, and thereby the warm-up operation of the engine is facilitated. Thereafter, when the coolant temperature becomes equal to or higher than the predetermined temperature, the thermostat is opened to circulate the coolant between the engine and the radiator. In this way, the coolant temperature is adjusted within an appropriate warm-up temperature range, and thereby the overheat of the engine is limited.
However, when an abnormality (known as a thermostat open state abnormality) occurs in the thermostat in a warm-up incomplete temperature range of the coolant, which is lower than the predetermined temperature described above, the thermostat is left opened. When the thermostat open state abnormality occurs, the coolant of the engine, which is in the middle of the warm-up operation, is circulated to the radiator and releases the heat through the radiator. Therefore, the temperature of the coolant in the radiator cannot be rapidly increased, and thereby completion of the warm-up operation of the engine is delayed. As a result, emissions of the engine may be disadvantageously increased, and the fuel consumption may be disadvantageously increased. Therefore, when the thermostat open state abnormality occurs, such an abnormality should be sensed in an early stage, and a warning should be provided to a driver (user).
In order to address the above disadvantage, as recited in, for example, Japanese patent No. 3407572 (corresponding to U.S. Pat. No. 6,279,390B1), the amount of change in an accurately measured coolant temperature, which is measured with a coolant temperature sensor, is compared with a determination reference temperature to determine whether the open state abnormality of the thermostat exists within a predetermined time period upon starting of the engine.
Also, as recited in, for example, Japanese Patent No. 3956663 (corresponding to US 2002/0111734A1), the coolant temperature is estimated based on the amount of coolant temperature increase caused by the generation of heat from the engine and the amount of decrease in the coolant temperature caused by the release of heat from the coolant through the radiator upon application of air flow, which is generated by the forward movement of the vehicle or a radiator fan and is applied to the radiator. Then, the estimated coolant temperature and the measured coolant temperature are compared with each other to determine whether the thermostat abnormality exists.
In the above abnormality diagnosis techniques, the coolant temperature, which is measured with the coolant temperature sensor, or the amount of change in the measured coolant temperature is compared with the estimated coolant temperature or the determination reference temperature to determine whether the coolant temperature shows the behavior of the normal time and thereby to diagnose the abnormality of the thermostat. In order to increase the accuracy of the abnormality diagnosis, the accuracy of estimation of the coolant temperature and/or the accuracy of the determination reference temperature should be increased. In order to increase the accuracy of the estimation of the coolant temperature and/or the accuracy of the determination reference temperature, the estimation method for estimating the coolant temperature and/or the determination reference temperature should be accurately adapted by measuring the amount of heat generated from the engine and the amount of heat released from the engine under various driving conditions and the traveling conditions of the vehicle through use of the actual vehicle. This adaptation disadvantageously requires a large number of steps.